Means for indicating the extent of a thermal treatment



Aprifi 2 134. w CHAPMAN I 2,1 5

MEANS FOR INDICATING THE EXTENT OF A THERMAL TREATMENT Filed Oct. 21, 1938 Patented Apr. 2, 1940 UNITED STATES MEANS FOR INDICATING THE EXTENT OF A THERMAL TREATMENT I Arthur WilliamChapman, Sheffield, England Application October 21, 1938, Serial No. 236,362

22 Claims.

This invention relates to thermally responsive liquids adapted to change permanently in colour as the result of undergoing thermal treatment.

Such thermally responsive'liquids have many applications in industrial and scientific fields where an indication is desired as to Whether or not some thermal treatment has been properly effected. For example, in the case ofdressings, instruments and the like for therapeutic work it is desirable to have an indication that sterilization has been thoroughly effected by the maintenance throughout the bulk of the body presumed to have been sterilized of adequate temperature for adequate time. Likewise, for example, in cold storage systems it is frequently desirable to have a permanent indication as to whether the temperature has or has not exceeded a certain value by an amount or for a time Which would be prejudicial.

The main object of the invention is to provide a thermally responsive liquid which will give an indication by a permanent observable change in colour if it has been subjected to a certain thermal treatment.

A further object of the invention is to provide a thermally responsive liquid which will give an indication by a permanent observable change in colour if it has been maintained at or above a certain temperature or within a certain temperature zone for a substantial or definite period of time.

A further object of the invention is the provision of a thermally responsive liquid which by its colour change indicates its thermal history.

A further object is the provision of a thermally responsive liquid which changes its colour when subjected to heat from an initial redcolour to green.

A further object is the provision diathermally responsive liquid which changes its colour progressively in time when subjected to heat from an initial red colour through amber or yellow to green.

Broadly, the thermally responsive. liquid according to the invention is an aqueous solution in which a chemical change takes place under certain heat treatment, such change resulting in a change of the hydrogen ion concentration, an indicator being included. in the solution to indicate visibly by change of colour the change of hydrogen ion concentration. The particular chemical change which is utilized in accordance with the invention is that which takes place between an ester in aqueous solution and the water.

For the purposes of this specification and the claims appended theretoan ester is defined as a compound derived from any alcohol or phenol and any acid such that the hydrogen of the acid is replaced by a hydrocarbon (or substituted hydrocarb-on) radical. The term ester includes for the purposes of this specification and the appended claims, esters of organic and inorganic acids, esters of halogen acids (i. e., alkyl halides) and esters of acid salts of polybasic acids, such for example as potassium ethyl sulphate.

Although under limiting conditions all esters as defined here will undergo reaction with water to yield the corresponding alcohol and acid, yet for the purposes of this invention all esters are not equally convenient and the use of members of certain classes of esters may be unsatisfactory in practice except in special applications. v

Compounds, such as chlorobenzene and phenyl benzoate, which are derived from phenols (which in these compounds play the part of the alcohol), may react too slowly with Water to be practically useful in most normal applications but they may have utility in special applications. The esters preferred, according to the invention, are therefore those derived from the alcohols.

Esters which are derived from very weak acids will liberate these acids when hydrolysed, but

unless the acid liberated is of sufiicient strength thechange in hydrogen ion concentration of the solution may be inadequate to produce in practice a clear and definite change of colour of a usual indicator, although some change of hydrogen ion concentration and some change of colour of a suitably sensitive indicator will generally occur after appropriate thermal treatment. For the purposes of this invention it is convenient that the acid produced should not be weaker than aceticv acid, the dissociation constant of which is approximately l-8 10- at 25 C. The esters preferred according to this invention are therefore those derived from acids of equalflor greater strength than acetic acid and having a dissociation constant not less than 1-8 10 at The chemical change or reaction which takes place between an ester and water results .in the liberation of the acid constituent of the ester accompanied by the reformation of the original alcohol of the ester. At any temperature this reaction is reversible, the formation of the acid and alcohol taking place concurrently with the re-combination of these products to form the ester and water. When, however, the initial state is, as in accordance with the. present invention, an aqueous solution of ester, with substantially no acid present, the forwardreaction producing acid and alcohol proceedsfaster than the recombination, whereby the net result is that acid and alcohol are produced until equilibrium is reached at which stage the rates of the reaction and the re-combination are equal. Before equilibrium is reached the net rate of production of acid and alcohol at any instant is a function of the temperature of a logarithmic nature. Thus, at and near the beginning of the hydrolysis of the ester, when the effect of the re-combination of the products of the hydrolysis may be neglected, the rate at which the hydrolysis proceeds may be expressed by a formula in the form:

7 Where It is the rate of progress of the reaction,

R is the gas constant,

T is the absolute temperature,

Z is a parameter of the reaction and is dependent upon, inter alia, the concentration of the solution,

E is a parameter of the reaction, and

e is the napierian logarithmic base.

Thus it may be seen that, below some Value of T, k is relatively small, but, above that value of T, It increases with increase of T relatively rapidly and at an increasing rate. Therefore it reaches a relatively large value at temperatures moderately exceeding that value of T. 'Hence, below that value of temperature T the net reaction proceeds relatively slowly,-and the aqueous solution may be regarded as relatively stable, While above that temperature the reaction proceeds relatively quickly to produce acid and thus to change the hydrogen ion concentration of the solution.

The rate of change of hydrogen ion concentration of the aqueous solution with respect to the rate of procedure of the main or primary reaction of the ester with the water to develop free acid and alcohol may be modified by the inclusion in the aqueous solution of a buffer salt which will enter into a secondary reaction with the acid produced by the primary reaction to produce a product less acidic than that acid, for example to produce a weaker acid. The efiect of the inclusion of a buffer salt is therefore to decrease the rates of change of hydrogen ion concentration with respect to absolute temperature and with respect to time. In the result the efiect of a buffer salt is to expand the range of time and temperature re quired to produce a certain definite change of colour of the indicator.

The general principles of operation of buffer salts are well known to chemists and the choice of a suitable buffer salt for any particular aqueous solution of an ester and any particular indicator may be made by the guidance of known principles. Typical buffer salts are disodium hydrogen phosphate, sodium borate and sodium acetate.

Some esters are only sparingly soluble in water and, as the invention requires an aqueous solution of an ester, the solubility of any particular ester may be increased by the addition of an auxiliary solvent'so that an adequate concentration of ester in the solution may be obtained. In general the auxiliary solvent takes no part in the reaction and merely serves the purpose of rendering the ester soluble to a sufficient extent in water. Various common commercial solvents may be employed such for example as acetone, methanol, ethanol or other alcohol, including, when appropriate, an alcohol corresponding with the ester utilized in the solution, or dioxan.

If it be desired to produce the particular sequence of colour change hereinbefore referred to, namely from red through amber or yellow to green, the choice of an indicator is of considerable importance. Initially the aqueous solution of an ester will have a relatively low hydrogen ion concentration but, after thermal treatment and consequent reaction of the ester with the water, the hydrogen ion concenration of the solution is increased. Now most otherwise suitable indicators are red, orange or colourless in solutions of higher hydrogen ion concentration whereas, in order to produce the colour change sequence referred to an indicator having at least a constituent which has a colour in acid solution towards the other end of the spectrum. (that is a non-red or a non-red-orange colour) is required. Water soluble aniline blue fulfils this condition and may therefore, in accordance with a feature of the invention, be employed as a constituent of the indicator.

Water soluble aniline blue may be used in combination with an indicating organic dye, which is red in alkaline solution, such as alizarin red 8., or one of the phenol red series. ever, both alizarin red S. and a dye of the phenol red series are used in conjunction with water soluble aniline blue and a water soluble yellow indicator dye such as brilliant yellow may also be added. The accompanying drawing is a longitudinal section of a device for indicating the extent of a thermal treatment in accordance with the invention.

The device comprises a transparentsealed tube 5 l, conveniently of glass, partially filled with the thermally responsive liquid 2.

A suitable quantity 2 of thermally responsive liquid prepared as subsequently described is sealed within the tube I, ample space 3 being left .with- Y in the tube to permit of expansion of the liquid.

In order to provide an indication as to whether particular given temperatures have been attained or not, bodies 4 .and 5, which are observably changed in state at definite pre-determined tem- V peratures, may be enclosed within sealed annexes G and l of the tube I. These bodies 4 and 5 may take the form'of pellets of compressed or cast materials or compounds each having a definite known melting point. bodies 4 or 5 may consist of a pellet of 4-nitro-2- amino-toluene, of which the melting point is between 104 C. and 105 C., or preferably, of catechol, of which the melting point is 104 C., with a very small trace of malachite green admixed therewith. The other body may conveniently comprise acetylsalicylic acid of which the melting point is C. Thus the melting and destruction of the shape of the first mentioned body indicates that, during the thermal treatment, a temperature of at least 104 C. has been attained While, if the second body is unmelted and preserves its shape, it is clear that a temperature of 135 C. had not been attained in the thermal treatment.

By including a tube I as described with a manufacture during the subjection of the manufacture to a thermal treatment and by permitting the device to remain associated with the manufacture after the thermal treatment until such time as the manufacture is required for use, a permanent indication is provided as to whether the manufacture has in fact been subjected thor oughly to the desired thermal treatment.

Preferably, how- For example one of the The xthermallyiresponsive liquid 2 comprises, 75

an-case's as before stated, essentially an aqueous solution of anester and an indicator but it may also com- "pri'se a buffer salt and an auxiliary solvent. The

ester 'is conveniently a compound 'ofan acid of which the dissociation constant is not less than '1-8 x 10- at 25 C. and of an alcohol and all the specific esters hereinafter referred to are em- "braced by this definition.

Ten thermally responsive liquids are hereinafter particularly described as examples only and not by way of limitation.-

In all the following examples the indicator solution referred to is prepared as follows:

. 0.5 gram of water soluble aniline blue is boiled with 100 cc. of water, cooled and filtered. 0.5 gram of alizarin red S. is boiled with 100 cc.-of water, cooled and filtered. 0.5 gram of brilliant yellow is dissolved in 100 cc. of water. 0.5 gram of chlorophenol red or 0.5 gram of phenol red is dissolved in 100 cc. of water with the aid of 1.5 cc.

of normal aqueous sodium hydroxide solution.

To 1 cc. of the aniline blue solution is added 1 cc. of normal aqueous sodium hydroxide solution. The mixture is dilute-d to about cc., boiled until the brown colour is nearly discharged and cooled. To this solution are then added 2 cc. of the alizarin red S. solution 1 cc. of the brilliant yellow solution and 1.5 cc. or" the chlorophenol red (or 0.15 cc. of the phenol red) solution and the mixture is diluted to 25 cc. to form the indicator solution.

' This indicator olution is red in alkaline solu- Example 1 In this example the ester utilized as the principal re-agent is an alkyl halide, namely ethylene chlorohydrin (Cl.CI-I2.CH2OH). This particular thermally responsive liquid is relatively stable at temperatures below about 100 C. but at higher temperatures and particularly about 115C the reaction proceeds relatively rapidly and the colour of the liquid changes from red through orange and yellow to green relatively quickly.

Therefore a tube l containing this thermally responsive liquid and containing if desired bodies 4 and 5 as described is particularly suitable for controlling the sterilization of surgical dressings and materials.

The thermally responsive liquid containing ethylene chlorohydrin is prepared as follows; 4.2 grams of ethylene chlorohydrin are added to 75 cc. of the buffer saltsolution hereinbeiore described, 10' cc. of the indicator solution hereinbefore described is added and the solution is made up to 100 cc. with Water.- I

At 115 C. this solution at the concentration mentioned. changes from red to a-yellow-green colour in from 20 to 25 minutes and changes to bright green in about30 minutes. At 100 C. the yellow-green colour is not reached until after about .100 minutes and the bright green colour is not reached until after about 110 minutes at that temperature and at room temperature the red colour. is retained for some years.

f "Ea'amplez In this example the ester utilized as the principal re-agent is potassium ethyl sulphate (C2H5KSO4), an ester of an acid salt of a poly- 5 basic acid. The thermally responsive liquid is made up as follows:

Cc. Aqueous solution containing 48 grams of crystalline potassium ethyl sulphate per 100 'cc 70 Buffer-salt solution Indicator solution -1 10 Water 100 I This thermally responsive liquid is relatively stable below about 130 C. but when heated in a sealed tube at a temperature from 150 C. to 155 C. it turns from red to green in colour in from to minutes.

Example 3 j In this example the ester utilized as the principal re-agent is an ester of an organic acid, namely methyl phthalate (C6H4(COOCH3)2).

This thermally responsive liquid is made up as follows:

Bufier salt solution 2 Water 3 Indicator solution 1 Absolute ethyl alcohol 4 Solution of methyl phthalate in absolute ethyl alcohol at a concentration of 10 grams of ester in 20 cc. of solution 2 This thermally responsive liquid is relatively stable below about 100 C., but when heated in a sealed vessel at a temperature of from 130 C. to 133 0. turns from red to green in approximately one hour.

Example 4 In this example the ester utilized as the principal re-agent is an ester of an inorganic halogen acid, (an alkyl halide) namely ethylene dibromide (Br.CHz.CH2Br). The thermally responsive liquid is made up as follows:

Bufier salt solution -1 1 Indicator solution. 1 Water 3 Acetone 5 Solution of ethylene dibromide in absolute ethyl alcohol at a concentration of 10 grams of the dibromide in 20 cc. of the solution 1 Example 5 ,In this example the ester utilized as the principal re-agent is an ester of an organic'acid, namely ethyl cyanoacetate ((CN) CI-IzCOOCzHs). This thermally responsive liquid is made up as follows:

a cc. Buffer salt solution 20 Indicator solution 10 Solution of the ester in absolute ethyl alcohol at a concentration of 10 grams of ethyl cyanoacetate in 50 cc. of solution 3 Water .100

Example 6 In this example the ester utilized as the principal re-agent is an ester of an organic acid, which is itself also an alkyl halide, namely ethyl chloroacetate (C1.CH2COOC2H5). This thermally responsive liquid is made up as follows:

cc. Buffer salt solution 4 Indicator solution 2 Water 8 Absolute ethyl alcohol 8 Solution of ethyl chloracetate in absolute ethyl alcohol at a concentration of 10 grams of ester in cc. of solution 2 This thermally responsive liquid is relatively stable below about 70 C. but at a temperature of 100 C. turns greenish in about 2 minutes and green in 4 minutes. However, if to 22 cc. of the liquid compounded as above mentioned, an addition of '1 cc. of bufier salt solution is made, the resultant liquid, when heated at 100 C. turns greenish in about 4 minutes and green in about 6 minutes. This demonstrates how the rate of change of hydrogen ion concentration of the solution with respect to the rate of procedure of the main or primary reaction of the ester with the water to develop free acid and alcohol may be modified by a modification of the bufier salt constituent.

Example 7 In this example the ester utilized as the principal re-agent is an ester of an organic acid, namely methyl oxalate ((COOCH3) 2). This thermally responsive liquid is made up as follows:

Buffer salt solution Indicator solution 10 Solution of methyl oxalate in dioxan, at a concentration of 25 grams of ester in cc. of solution 1 Water 100 This liquid re-acts and turns green more quickly at lower temperatures than those heretofore mentioned. At room temperature the solution turns from red to green in a few hours. At a temperature of about 90 C. it turns greenish in 1 minute and green in 1 /2 to 2' minutes.

Example 8 In this example the ester utilized as the principal re-agent is an ester of an organic acid, namely ethyl formate (H.COO C2H5). This thermally responsive liquid is made up as follows:

cc. Buffer salt solution 10 Indicator solution 10 Ethyl formate 4 Water 100 Example '9 In this example the ester utilized as the principal re -agent is an ester of an organic acid, namely ethyl oxalate (COOCzI-Is) 2) This thermally responsive liquid is made up as follows:

Again this liquid reacts quickly at a relatively lower temperature turning greenish in about 4 minutes and green in about 5 minutes at a temperature of 70 C. to 80 C. At room temperature, however, it takes several hours to turn green. This liquid is useful in connection with chill storage of meat and food and serves to indicate whether or not the meat or food has exceeded a safe temperature for a period of more than an hour or so. For this use the indicator described may be replaced by one which is red in acid solution.

Example 10 In this example the ester utilized as the principal reagent is methyl tcluene-p-sulphonate Solution of methyl toluene-p-sulphonate in absolute ethyl alcohol at a concentration of 10 grams of ester in 20 cc. of solution 2 This liquid turns green at a temperature of C. to C. in about 6 minutes but at temperatures below about 30 C. takes a relatively long time to change its colour completely.

It is useful in connection with the cold storage of meat and food and serves to indicate whether or not the meat or food has exceeded a safe temperature for a period of time. For this use the indicator described may be replaced by one which is red in acid solution so that the red danger signal is produced if the liquid has been subjected for a material time to a temperature in excess of that desirable.

This application is a continuation-in-part of my application, Serial No. 84,216, filed June 8, 1936.

What I claim is:

1. A thermally responsive liquid solution comprising Water, an ester of an acid having a dissociation constant not less than 1-8 10- at 25 C., a buffer salt and an indicator which alters in colour with change of hydrogen ion concentration of the liquid.

2. A thermally responsive liquid solution comprising water, an ester of an acidhaving a dissociation constant not less than 1-8 10 at 25 C. and of an alcohol, a buffer salt and an indicator which alters in colour with change of hydrogen ion concentration of the liquid.

3. A-thermally responsive liquid solution comprising water, an ester of an inorganic acid having a dissociation constant not less than 1 -8 X 10- at 25 C. and ofan alcohol, a buffer salt, and an indicator which alters in colour with change of hydrogen ion concentration of the liquid.

4. A thermallyxresponsive liquid solution comprising water, an ester of a halogen acid and of an alcohol, a buffer salt, and an indicator which alters in colour with change of hydrogen ion concentration of the liquid.

5. A thermally responsive liquid solution comprising water, an ester of hydrochloric acid and of an alcohol, a buffer salt, and an indicator which alters in colour with change of hydrogen ion concentration of the liquid.

6. A thermally responsive liquid solution comprising water, ethylene chlorohydrin, a buffer salt, and an indicator which alters in colour with change of hydrogen ion concentration of the liquid.

7. A thermally responsive liquid solution comprising water, an ester of an organic acid having a dissociation constant not less than l-8 10- at 25 C. and of an alcohol, a buffer salt, and an indicator which alters in colour with change of hydrogen ion concentration of the liquid.

8. A thermally responsive liquid solution comprising water, methyl phthalate, a buffer salt, and an indicator which alters in colour with change of hydrogen ion concentration of the liquid.

9. A thermally responsive liquid solution comprising water, an ester of an acid salt of a polybasic acid, said acid salt having an acid dissociation constant not less than l-8 10-- at 25 C. and of an alcohol, a. bufier salt, and an indicator which alters in colour with change of hydrogen ion concentration of the liquid.

10. A thermally responsive liquid solution comprising water, potassium ethyl sulphate, a bufier salt, and an indicator which alters in colour with change of hydrogen ion concentration of the liquid.

11. A thermally responsive liquid solution comprising water, an ester of an acid having a dissociation constant not less than 1-8 10- at 25 C. and of alcohol, disodium hydrogen phosphate and an indicator which alters in colour with change of hydrogen ion concentration, of the liquid.

12. A thermally responsive liquid solution comprising water, ethylene chlorohydrin, disodium hydrogen phosphate and an indicator which alters in colour with change of hydrogen ion concentration of the liquid.

13. A thermally responsive liquid solution comprising water, ethylene chlorohydrin, disodium hydrogen phosphate and an indicator comprising water soluble aniline blue, alizarin red S, brilliant yellow and an indicating dye of the phenol red series.

14. A thermally responsive liquid solution comprising ethyl alcohol, water methyl phthalate, disodium hydrogen phosphate and an indicator comprising water soluble aniline blue, alizarin red S, brilliant yellow and an indicating dye of the phenol red series.

15. A thermally responsive liquid solution comprising water, potassium ethyl sulphate, disodium hydrogen phosphate and an indicator comprising water soluble aniline blue, alizarin red S, brilliant yellow and an indicating dye of the phenol red series.

16. A thermally responsive liquid solution comprising water, an ester of an acid having a dissociation constant not less than 1-8 10- at 25 C. and of an alcohol, a buffer salt, and an indicator comprising water soluble aniline blue.

17. A thermally responsive liquid solution comprising water, an ester of an acid having a dissociation constant not less than 1-8 10- at 25 C. and of an alcohol, a bufi er salt, and an indicator comprising water soluble aniline blue and an indicating organic dye of the group which is red in alkaline solution and yellow in acid solution.

18. A thermally responsive liquid solution comprising water, an ester of an acid having a dissociation constant not less than 1-8 10-- at 25 C. and of an alcohol, a buffer salt, and an indicator comprising water soluble aniline blue and alizarin red S.

19. A thermally responsive liquid solution comprising water, an ester of an acid having a dissociation constant not less than 1-8 10- at 25 C. and of an alcohol, a bufier salt, and an indicator comprising water soluble aniline blue, alizarin red S, and brilliant yellow.

20. A thermally responsive liquid solution comprising water, an ester of an acid having a dissociation constant not less than 1*8 10 at 25 C. and of an alcohol, a bufier salt, and an indicator comprising water soluble aniline blue, and an indicating dye of the phenol red series.

21. A thermally responsive liquid solution comprising water, an ester of an acid having a dissociation constant not less than 1-8 "10- at 25 C. and of an alcohol, a bufier salt, and an indicator comprising water soluble aniline blue, alizarin red S, brilliant yellow and an indicating dye of the phenol red series.

22. A thermally responsive liquid solution comprising water, an ester of an acid having a. dissociation constant not less than 1-8 10- at 25 C. and of an alcohol, which ester is sparingly soluble in water, an auxiliary solvent adapted to dissolve and to increase the solubility of the ester, a bufier salt, and an indicator which alters in colour. with change of hydrogen ion concentration of the liquid.

ARTHUR W. CHAPMAN. 

